I purchased a Remington Model Seven that was sold as a 300SAUM. It has a factory barrel that originally had iron sights (can see where the tapped holes were filled) and the barrel has been fluted (and in doing so the caliber designation must have been obliterated) and will NOT chamber a 300SAUM. The bolt is a SAUM boltface, and the best I can mic the muzzle is with my digital gauge is .270 so I think it may be a 7mmSAUM. what do I need to do to be absolutely sure? Cast the chamber with Cerrosafe?

Chamber cast would tell the story.

Can I cast the chamber while the barrel is still on the action or does it need to be pulled?

It can be done without pulling the barrel.

Could also be a 6.5 rem mag

Not many more options for a factory barrel

It measures larger than 6.5 at the muzzle. I have some cerrosafe ordered.

Changed my mind and took it to a smith today. Figure at least it looks better to the seller if a smith casts the chamber vs me doing it (he has offered to at minimum pay for chamber casting to determine caliber). Should have the rest of the story by mid week next week.

270 WSM?

270 SAUM ?

Got it back from the smith today and it is a 7mmSAUM. He scoped the barrel and said the rifling and chamber all look great and it appeared to have been shot very little.

Going to keep it as is?

Yes, the barrel seems to have good life left so I will keep it as is. I also just purchased a 700 in 300SAUM (with the factory stamping)

Now for the hard part 7mm SAUM ammo or brass.

Did he stamp it for you or is is still an "unknown" for the future?

A little strange on that one...This smith did not have the equipment to stamp the barrel. He has a smaller shop but it seemed odd that he had the means to cast a chamber (and he gave me the casting) but not to stamp a barrel. I will just order a stamp set and do it. Been needing an excuse to get a set anyway. I got my cerrosafe in yesterday so if that need ever arises again I will have some on hand.

For sure, 7mm is definitely harder to find vs 300 (and that’s not just everywhere especially now)

Making a chamber cast is not rocket science.

Would not think stamping a barrel is either

IIRC ADG are doing 7mm SAUM brass.
Not saying it's available, but at least it is listed.
https://www.creedmoorsports.com/product/adg-7mm-saum-brass/ADG-Brass

How hard it it to hit a stamp with a hammer?

Good to know. Thanks!

shoudn't be hard to form brass from 300 saum

That sounds like a really fun puzzle to have solved!